Modification of the electrical properties of polystyrene and other polymers by introduction of inorganic reinforcements (e.g., carbon black, iron powder) has been long pursued by researchers1. Recently, the use of multi-walled carbon nanotubes (MWNTs) has been demonstrated as more advantageous than carbon black, not only because of the need of smaller loads to reach the conductivity threshold, but also because their high aspect ratio helps create extensive networks that facilitate electron transport. Single-walled carbon nanotubes (SWNTs) have not been studied as extensively as MWNTs in applications involving conductive nanotube-filled polymers. One reason for this is that strong attractive Van der Waals forces lead them to easily aggregate, and thus, with the prior art cast techniques, it is very difficult to form conductive networks at relatively low loadings.
Additionally, the reason that engineering research aimed toward the purification, functionalization and incorporation of carbon nanotubes into polymeric matrices has been actively pursued recently is that vastly important applications, from anticorrosion paints to nanometer-thick conductive thin films, are expected to have a direct impact on a world market worth hundreds of millions of dollars. Among the challenges introduced in the fabrication of nanotube-filled polymer composites is the necessity to creatively control and make use of surface interactions between carbon nanotubes and polymeric chains in order to obtain an adequate dispersion throughout the matrix without destroying the integrity of the carbon nanotubes (NTS). Solution-evaporation methods have been the major experimental route to prepare both filled thermoplastics such as polystyrene (PS)2, polyvinyl alcohol (PVA)3, polyhydroxyaminoether (PHAE)4,5, poly(m-phenylenevinylene-co-2,5-dioctyloxy-p-phenylenevinylene) (PmPV)6, as well as epoxy thermosets7. Solution-casting methods have limited applicability for producing highly conductive films because SWNT composites tend to saturate at 1.2% nanotube content as the excess nanotubes aggregate5. In addition, most polymeric materials need a large amount of solvent to be completely solubilized and to consequently incorporate the NTs. Moreover, these solvents are known organic liquids of high toxicity such as toluene, chloroform, tetrahydrofurane (THF) or dimethyl formamide (DMF) therefore their use is to be avoided.
Recently, inorganic fillers such as carbon black8, titanium dioxide9, magnetite10, calcium carbonate9,10, and silica11 have been encapsulated by using the technique of surfactant-enhanced miniemulsion polymerization. In miniemulsion polymerization, the inorganic filler is first dispersed in a monomer and then the mixture is subjected to conditions of high shear typical of miniemulsification. Use of oil-soluble or water-soluble radical initiators under mild temperature conditions accelerates the polymerization, originating hybrid materials with different structures, e.g., encapsulated filler; polymer core with attached inorganic particles; or a mixture of both11. A similar procedure to create a doped polyaniline (PANI)-Multiwall carbon nanotube composite under conditions of emulsion polymerization was done by Deng et al12 who observed an increase in electrical conductivity of about 25 times between the PANI-NTs composite (10% load of NTs) with respect to the conductivity of the parent PANI. Another example of surfactant-aided composite fabrication employs a dispersion of carbon nanotubes in a cationic surfactant to generate co-micelles which serve as a template for the synthesis of nanotube-containing silicon oxide micro-rods13. However, in situ miniemulsion techniques have not been used to combine SWNTs with monomers to form SWNT-composites.